Television picture signals contain random noise from a variety of sources. These sources include electronic noise generated within the transmitter, within the receiver and in the environment between the transmitter and the receiver. Many television signal receivers contain noise reduction circuits that may be adjusted to reduce the level of noise on the received signal without adversely affecting the processing of the received television signal. Often, this noise reduction circuit is manually adjusted to a level of noise reduction corresponding to an average level of expected noise. When the noise received at a television receiver is more than expected, or less than expected, the manually adjusted noise reduction system will not be optimally adjusted. An automatic noise measurement system may provide a means of controlling the noise reduction circuit in a television receiver so that low levels of noise reduction could be applied during the periods of low noise and high levels of noise reduction could be applied during periods of high noise.
Storey (U.S. Pat. Nos. 4,249,209 and 4,249,210, issued Feb. 3, 1981) provides for a video noise reduction system that utilizes successive scans of the television signal to produce a difference signal. This difference signal is then applied to the noise reduction circuit. The picture information is chosen for successive scans. When the difference between the successive lines is too large, it is assumed that there is motion in the picture. When the difference is small, it is assumed that the difference is due to the noise in the signal. The noise signal is then scaled and passed to a noise reduction circuit. The system as provided by Storey is not able to continuously measure the noise that exists in a signal where the picture is changing rapidly.
Balbes et al. (U.S. Pat. No. 4,189,755 issued Feb. 19, 1980) provide for measuring noise on the sync tip of a composite television picture signal by clamping the sync tip signal to ground and then applying the clamped sync tip signal to a precision rectifier. The output of the rectifier represents the noise level in the signal and the level of the television signal is determined from the magnitude difference between the sync tip and the back porch. A signal-to-noise ratio signal is generated and the bandwidth of the receiver is adjusted in response to the signal-to-noise ratio signal. If there is a color burst signal on the back porch, it is removed by a filter that may also remove part of the noise to be measured. Neither Storey nor Balbes et al. determine noise levels by comparing the back porch signal (or the color burst signal) from one scan line to the same signal from another scan line.